If you have ever been in the fittings aisle holding a threaded pipe in one hand and an entire wall of adapts in the other, then you understand the dilemma that the NPT size chart helps to solve. The problem is that the NPT threads from one size to another all look the same, and that the size of the fitting does not correspond to the actual diameter of the size in inches that you can measure accurately with a rule. For example, an NPT fitting that is labeled as “1/2 inch” NPT fitting only has ut 84” across its threads — not because the “1/2 inch” in question refers to the nominal pipe size of the obsolete iron pipe standard, but because it is the thread diameter being referred to here.
The NPT Size chart exists so that you can find out the actual measurements for the real diameter in inches, threads per inch, pitch, and the size of the tap drill that will be required to tap the fitting so that you can have an understanding of what the size will actually be.
NPT (National Pipe Taper) reference sizes vary from 1/16 inches to 6 inches and above, and the identification of its name corresponds to the nominal pipe size not its actual measured thread diameter. The critical data elements that adjoin this reference size are as follows: outside thread diameter, threads per inch (TPI), and the tap drill size for any internal threads.
What Is NPT and How the Size System Works
NPT, or National Pipe Taper, is the U.S.A. standard for tapered pipe threads, and it is regulated by ASME B1.20.1. The NPT thread is made with a one in sixteen taper (which means that the diameter decreases by 3/4" for every foot of the length), meaning that male and female threads lock together as the turning process continues, and the interference created by the two threads makes the seal with the assistance of PTFE tape or thread sealant.It belongs to the same fitting family as the sanitary connections covered in our sanitary fitting guide, which explains how threads and clamp connections compare across systems.
The naming of sizes causes historical confusion. NPT sizes are nominal pipe sizes (NPS) of the wrought-iron piping standard used to make the thread, but they do not indicate actual thread diameters. For example, a 1/2" NPT thread has a major diameter of about 0.840", while a 3/4" NPT thread measures approximately 1.050". Fortunately, the correlation between NPT size and diameter is thoroughly documented; which is why a size chart exists.
The Complete NPT Size Chart (1/16" to 6")
| Nominal Size | Threads per Inch (TPI) | Pitch (mm) | External Thread Major OD (in) | External Thread Major OD (mm) | Tap Drill (in, for internal threads) | Tap Drill (mm) |
|---|---|---|---|---|---|---|
| 1/16 | 27 | 0.941 | 0.3125 | 7.94 | 1/4 | 6.35 |
| 1/8 | 27 | 0.941 | 0.405 | 10.29 | 21/64 | 8.33 |
| 1/4 | 18 | 1.411 | 0.540 | 13.72 | 7/16 | 11.11 |
| 3/8 | 18 | 1.411 | 0.675 | 17.15 | 9/16 | 14.29 |
| 1/2 | 14 | 1.814 | 0.840 | 21.34 | 11/16 | 17.46 |
| 3/4 | 14 | 1.814 | 1.050 | 26.67 | 7/8 | 22.22 |
| 1 | 11.5 | 2.209 | 1.315 | 33.40 | 1-3/32 | 27.78 |
| 1-1/4 | 11.5 | 2.209 | 1.660 | 42.16 | 1-13/32 | 35.72 |
| 1-1/2 | 11.5 | 2.209 | 1.900 | 48.26 | 1-5/8 | 41.28 |
| 2 | 11.5 | 2.209 | 2.375 | 60.33 | 2-1/32 | 51.59 |
| 2-1/2 | 8 | 3.175 | 2.875 | 73.03 | 2-1/2 | 63.50 |
| 3 | 8 | 3.175 | 3.500 | 88.90 | 3-1/16 | 77.79 |
| 3-1/2 | 8 | 3.175 | 4.000 | 101.60 | 3-9/16 | 90.49 |
| 4 | 8 | 3.175 | 4.500 | 114.30 | 4-1/16 | 103.19 |
| 5 | 8 | 3.175 | 5.563 | 141.30 | 5-1/16 | 128.59 |
| 6 | 8 | 3.175 | 6.625 | 168.28 | 6-1/16 | 153.99 |
Chart notes: The tap drill sizes are standard drill sizes for making the first holes when using a taper tap (internal cutting of NPT (national pipe threads) threads) and are approximate values; use a pipe-tap drill table or manufacturer’s recommendation for exact value. The pitch column provides measurements (distance between thread crests) which may be verified with a thread gauge. Measurements are given according to ASME B 1.20.1 standards.
How to Read the Chart: OD, TPI, Pitch, Tap Drill
Four criteria exist:
- Threads per inch (TPI): determine the number of thread peaks in an inch using a thread pitch gauge. This will yield the quickest identification via 27 TPI representing either 1/8" or 1/16", 18 TPI representing either 1/4" or 3/8", 14 TPI representing either 1/2" or 3/4", 11.5 TPI representing between 1" – 2" and 8 TPI representing 2-1/2" and greater.
- External thread major bearing diameter (OD): the largest diameter of the male thread, which is measured at the first (or largest) thread — this is an actual measurement taken with using a caliper, it will differentiate the same-TPI sizes (1/4" at 0.540" compared to 3/8" at 0.675").
- Pitch (mm): the distance from peak to peak, essentially the metric equivalent of TPI and used only with metric gauges.
- Tap drill: the drill bit used to cut the internal (female) thread — the hole must be slightly larger than the minor diameter of the thread in order to allow for the taper tap to cut the thread.
The sequence of identification involves measuring the OD using a caliper first, second confirming the TPI using a gauge, and finally, determining the row, whereby the OD will separate same-TPI sizes; TPI will then allow the differentiation between sizes of the same TPI that have similar ODs.
Pipe Size vs Tube Size: The Confusion Explained
Layer of confusion number two: Pipes and tubes do not belong to the same system, with respect to size. NPT — the designation of a pipe thread — is a nominal size and describes the pipe's approximate internal diameter. Therefore, the outside diameter of the pipe in question would have a measurement higher than its designation (the outside diameter on a 1/2" pipe is 0.84" OD). Tube sizes are designated according to their outside diameter: therefore, a 1/2" tube is measured as 0.50" OD by definition.This is why a 1/2" tube fitting and a 1/2" pipe fitting are completely different sizes, and why a tube adapter to NPT must be specified by both the tube OD (in inches or mm) and the NPT size. In sanitary and brewing work, for example, tube is measured by actual OD (19 mm, 25 mm, 38 mm), while the NPT ports on pumps and valves are named by pipe size — the adapter bridges the two systems, and the measurement rules for sanitary tube connections are covered in our sanitary fittings measurement guide.
NPT to AN Conversion: The Aviation Sizing System
AN fittings (Army-Navy, now governed by SAE AS) are the aviation/hydraulic thread and hose system, and the AN number is simply equal to 16 times the outside diameter of the tube in inches. The conversion between AN size and its equivalent pipe/tube diameter is continuously one of the biggest challenges people face; thus this is the table:
| AN Size | Tube OD (in) | Tube OD (mm) | Closest NPT / Pipe Size |
|---|---|---|---|
| -4 AN | 1/4 | 6.35 | 1/8 NPT (close) |
| -6 AN | 3/8 | 9.53 | 1/4 NPT |
| -8 AN | 1/2 | 12.70 | 3/8 NPT |
| -10 AN | 5/8 | 15.88 | 1/2 NPT |
| -12 AN | 3/4 | 19.05 | 1/2-3/4 NPT range |
| -16 AN | 1 | 25.40 | 3/4-1 NPT range |
| -20 AN | 1-1/4 | 31.75 | 1 NPT |
| -24 AN | 1-1/2 | 38.10 | 1-1/4 NPT (close) |
Important: AN and NPT represent distinctly different types of threads (whereas the AN uses a 37-degree flare and straight UNF thread, NPT is a tapered thread with a 60-degree taper) thus, the two cannot be interchanged directly. The table shows the equivalency of flow path (the size of the AN hose that fits into size NPT pipe), thus a fitting will always have to be used whenever transitioning between the two. When ordering fittings make sure you specify each side precisely: for example, "-8 AN male to 3/8" NPT female." The cost side of sourcing adapters and fittings follows the same pattern as the rest of the tri-clamp and threaded system — our fittings replacement cost analysis shows how factory-direct pricing compares across configurations.
NPT vs NPTF vs BSPT: The Related Standards
The standard shown in this chart is as follows:
- NPT (ASME B1.20.1): The standard tapered pipe thread used for general applications (sealing with tape or compound).
- NPTF (dryseal) (ASME B1.20.3): This is a modified version of NPT that uses controlled crests and roots so that it seals without using tape or compound through metal-to-metal interference. This is primarily used for hydraulic applications and although NPT and NPTF are dimensionally similar, they cannot be used interchangeably for leak-free connections.
- NPS / NPSM: Thiss is the straight pipe thread.
- BSPT (British Standard Pipe Taper): The UK tapered pipe thread (it is not interchangeable with NPT since BSPT has a 55° thread angle compared to 60° for NPT as well as other dimensional differences). A fitting with a 1/2" BSPT will start on a 1/2" NPT fitting but will cross-thread in one or two turns. Always check what type of thread a fitting uses before forcing them together since the difference in angle will cause damage if they are forced.
The other standards that appear in the same catalogs — and how they relate to the NPT dimensions above — are covered in our FIP vs NPT guide, with the female-thread terminology in the FNPT meaning guide.
How to Measure an NPT Thread Correctly
How to identify an NPT fitting in your hand:
- Take the measurement of the outside diameter of the male thread with a caliper at the largest thread (the first thread). This gives you the major OD in the chart. For female fittings, the measurement is taken with the internal diameter of the largest thread.
- Determine TPI using a thread pitch gauge or count the crests over one inch; this gives you the right size. To clarify: 27 TPI = 1/8” class, 18 TPI = 1/4” and 3/8”, 14 TPI = 1/2” and 3/4”, 11.5 TPI = 1” to 2”, and 8 TPI = over 2-1/2”.
- Now match the OD from #1 to your row: 0.405” + 27 TPI = 1/8”; 0.840” + 14 TPI = 1/2”; etc.
- Confirm the taper of the pipe fitting by seeing that NPT threads are tapered (they change diameter) as opposed to parallel threads. A simple method of checking this would be to measure the OD at the start of one thread and the end of the last thread. Remember that tapered pipes will change about 3/4” for every foot.
If you are unsure if the fitting is NPT or it is NBR, be sure to take the thread angle measurement: 60° will be NPT, while 55° is NBR. Whenever you take a connection apart, be sure not to force it, as cross-threading different standards destroys them both.
Frequently Asked Questions
What is the standard NPT thread size range?
NPT threads are available in range of sizes from 1/16" (0.3125" OD, 27 TPI) to 6". The common range for NPT Threads is between 1/8" to 2" (2.375" OD, 11.5 TPI). The larger utility sizes have a standard of 8 TPI.
Is 1/2 NPT the same as 1/2 BSP?
1/2 NPT and 1/2 BSPT are not interchangeable. They both have the size designation of "1/2"" and both utilize 14 TPI. However, NPT threads are created with a 60° thread angle and have differing diameters (for example the 1/2" NPT measures 0.840") whereas BSPT is made of a 55° thread angle and has a different diameter (for example the 1/2" BSP measures 0.825"). They will match for a brief moment, then cross-thread. Be sure to identify which standard you are using prior to assembly.
What is the AN equivalent of NPT sizes?
The AN size is determined by multiplying the tube OD in inches by 16. For example: -4 AN equals 1/4" tube; -6 AN equal 3/8"; -8 AN equals 1/2"; -10 AN equals 5/8"; -12 AN equals 3/4"; -16 AN equals 1"; -20 AN equals 1 1/4"; and -24 AN equals 1 1/2". The flowing equivalents would be approximately: -6 AN = 1/4" NPT; -8 AN = 3/8" NPT; -10 AN = 1/2" NPT. Note that AN and NPT thread forms are different and therefore requires the use of an adapter.
What is the difference between pipe size and tube size?
Pipe Size (NPS/NPT) is a nominal size based upon the legacy size of the piping product as indicated by the inside diameter of the thread. In each case the OD of the thread always exceeds the name (thus 1/2" pipe thread measures 0.840"). Tube size refers to the actual outside diameter of the tube — thus a 1/2" tube has a dimension of 0.50".
References
- ASME B1.20.1 — Pipe threads, general purpose (inch), NPT dimensions
- NFPA — Pipe and fitting standards references
- ANSI — National pipe thread standards
- ThreadSpec — NPT thread dimensional data
- Machining Doctor — NPT thread data and identification tools
Conclusion
The NPT size chart serves as the interpretive bridge between what the fitting is called and what it is you actually possess: nominal sizes that do not correspond to their actual measurements, TPI that denotes the size category, and pitch and tap-drill numbers that allow the user to cut and identify the threads without guesswork. Essential information is limited to — for example 1/8 inch is equal to 0.405 inch at 27 TPI, 1/2 inch is equal to 0.840 inch at 14 TPI, 1 inch is equal to 1.315 inch at 11 TPI, and 2-1/2 inch and larger steps to 8 TPI — and the adjacent systems of pipe vs. tube, NPT vs. BSPT and NPT vs. AN each have one rule: pipe size is nominal, tube size is true OD, BSPT is defined by 55-degree angle and AN is 16 × tube OD. The FNPT to English guide completes the understanding of female-thread definitions, and with a chart in hand, you should be able to measure the OD, check TPI and verify the taper every time — making sure the plumbing connection, sanitary line or hydraulic coupling will not become a leaking mess tomorrow.
